In a vehicle engine, feedback control of a fuel-air ratio of air-fuel mixture aspirated into a combustion chamber based on oxygen density in the exhaust is disclosed for example in Tokkai Sho 60-101243 published by the Japanese Patent Office in 1985. Specifically, an injection amount of a fuel injector injecting fuel into an intake port of the engine is controlled based on the oxygen density in the exhaust. The fuel-air ratio is a reciprocal (1/.lambda.) of the air-fuel ratio (.lambda.).
However when the load of an auxiliary instrument such as an air conditioner acts on the engine during idle running (viz., idling), to maintain the engine rotating speed of an engine at a predetermined limit necessary to maintain stability of combustion, the fuel supply amount must be increased to increase the output torque of the engine.
Due to this control, the intake air amount and fuel amount aspirated by the engine increase together, but as air is a compressible fluid, increase of air inflow to the combustion chamber is relatively gradual compared to the increase in the opening of the intake throttle. On the other hand, as part of the fuel injected from the fuel injector adheres to the surface of the port wall, the fuel inflow amount to the combustion chamber of the engine increases slowly relative to increase of injection amount.
In a multi-cylinder engine immediately after torque increase control, fuel oversupply or undersupply may occur in cylinders depending on the combustion sequence, and the air-fuel ratio is apt to change between rich and lean. A rich shift of the air-fuel ratio acts to stabilize combustion if it is within a certain range, but a lean shift of the air-fuel ratio may make combustion unstable.